Kernals of shelled corn are milled into meal and flour by two general systems, namely, wet milling and dry milling.
Conventional wet milling methods are described in U.S. Pat. Nos. such as 2,584,893; 2,704,257; and 3,083,103. In these and other conventional wet processes, whole kernals of shelled corn are soaked and steeped in hot lime water until the kernals have been completely penetrated by the hot alkaline solution, resulting in the kernal becoming softened and the hulls partially digested. The lime treatment was required to soften and digest and hemi-cellulose content of the hulls and to peptize or disrupt the protein content of the corn kernal so that the kernals and hulls could be more readily ground.
Steeping corn kernals in hot lime water is a lengthy process ranging from a few hours to as much as 24 hours. Furthermore, the steeping is followed by washing away the lime water before grinding the corn. This results in extraction and wasting of the thiamin, riboflavin, and niacin content of the whole corn. Other alkaline-soluble nutrients and protenaceous materials are also lost by this process. This loss due to steeping and washing is not only wasteful from a nutritional standpoint, but also results in a loss of total yield, which of course is an economic loss. Moreover, where the product is to be dried to a flour for later reconstitution to a dough, an economic disadvantage is encountered because of the energy required to evaporate the water added to the product during the soaking and steeping operations.
Another disadvantage of the wet process is that the lime treatment imparts a characteristic flavor which is different from whole corn flavor and which is found objectionable by many consumers.
In dry milling, whole kernals of corn, not subjected to the lime water steeping just described, have been ground into cornmeal and corn flour since colonial times. Such meal and flour is preferred because it has a better flavor than the wet-processed product, and contains substantially all of the nutrition naturally present in whole kernals. However, it has storage stability problems. Mature corn kernals are composed of four major parts: the pericarp (hull or bran), germ (embryo), endosperm, and tip cap. The germ is very nutritious because it contains a large portion of the protein and about 85% of the total corn oil (lipids) in the kernal. When the raw whole kernals of corn are ground, the lipids released from the germ apparently come in contact with certain enzymes in the corn kernal, causing rapid onset of hydrolytic rancidity. Longer storage time adds oxidative rancidity, which is caused by air oxidation of unsaturated fatty acids in the corn oil. Although a small amount of fatty acids is part of the desirable corn flavor, an excess causes bitter flavors in the food, and can make it inedible.
Since whole cornmeal and flour contains all of the original oil, its shelf-life, when sold in packages, is considerably reduced. Attempts to lengthen the shelf-life of whole cornmeal or flour by the use of heat or antioxidants have so far been unsuccessful.
Consequently, most modern-day cornmeal or flour which is to be stored for even a short length of time is prepared from degerminated corn kernals, which are free of the hull and germ. The degermination process is not only time consuming and expensive, but it also removes from the corn an important part of the nutrition in the whole kernel.
U.S. Pat. No. 3,404,986 describes a process for preparing corn flour from cornmeal which is soaked in water before processing into flour. This product does not have a long shelf-life, and requires combining with flour made from degerminated cornmeal.
U.S. Pat. Nos. 3,694,220 and 3,701,670 describe a process for preparing feed from corn kernels heated by infrared radiation for a relatively short period of time, and then passed through a rolling mill to form flakes. This method is not satisfactory for preparing flour for human consumption because the final product contains hard "flinty" particles.